The present invention relates in general to DC-DC converters in electric drive systems for electrified vehicles, and more specifically to a circuit topology of a switching DC-DC converter achieving a high voltage gain using relatively small component sizes and low power losses.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system may include a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a DC-DC converter (also known as a variable voltage converter, or VVC) to regulate a main bus voltage across a main DC linking capacitor. A 3-phase motor inverter is connected between the main buses and a traction motor in order to convert the DC bus power to an AC voltage that is coupled to the windings of the motor to propel the vehicle. During deceleration of the vehicle, the motor can be driven by the vehicle wheels and used to deliver electrical power to charge the battery during regenerative braking of the vehicle, wherein the DC-DC converter works in the opposite direction to convert the generated power to a voltage appropriate for charging the battery pack. In some vehicles, another 3-phase inverter may also be present to connect the DC bus to a generator which is driven by an internal combustion engine to charge the battery or provide power to the motor.
Using the appropriate modulation of the power switches, a VVC can operate in boost mode (converting to a higher voltage), buck mode (converting to a lower voltage), or pass-through mode (no change in voltage). For use in a hybrid electric vehicle driver system, the VVC is also configured to selectably provide bi-directional power flow.
The typical VVC includes at least one phase leg with upper and lower transistor switching devices (e.g., insulated gate bipolar transistors, IGBTs) connected in series across the DC link capacitor. An intermediate junction between the switching devices is connected to the source battery via an inductor. An electronic controller provides switching signals (i.e., gate signals) to turn the switching devices on and off according to a modulation scheme that provides the desired VVC mode. Pulse width modulation is typically used to control the stepping up of a voltage by the VVC, wherein a duty cycle of the switching signals can be varied in order to regulate the VVC voltage to a desired magnitude.
The voltage gain of conventional DC-DC converters have been limited and/or power losses when providing a very high gain have been significant. For the typical circuits, the gain is determined by a duty cycle D defined as Ton/Ts, where Ton is the conduction duration of the lower switching device and Ts is the switching period. Based on the duty cycle, the voltage gain G is determined by the formula
  G  =                    V                  d          ⁢                                          ⁢          c                            V        b              =                  1                  (                      1            -            D                    )                    .      The converter efficiency dramatically decreases with increasing the duty cycle D when voltage gain G is larger than two. Consequently, the voltage gain for conventional DC-DC converters has typically been limited to less than three. Higher voltage gains would be desirable to reduce motor inverter loss over a wide speed range operation. In addition, operating the DC-DC converter at a higher duty cycle for most of the time results in higher power loss and high voltage stress within the phase leg switching devices. Therefore, an improved variable voltage converter is needed that can provide higher voltage gain at reduced duty cycles.
Another potential drawback of conventional interleaved converters is that high current ripple in the inductors creates a larger power loss when the duty cycle D is high. Large inductors have been necessary to limit current ripple, but they are lossy, bulky, and heavy which is undesirable for high power HEV applications.